1. Field of the Invention
The present invention relates to an electro-optical display containing an electrochromic material which manifests reversible variations in the light absorption properties upon application of properly controlled voltage or current. This display is referred to as an "electrochromic display (ECD)" hereinafter.
The present invention relates, more particularly, to a manufacturing method for an electrochromic display cell which employs a thin film of amorphous tungsten (WO.sub.3) as an electrochromic material, and a liquid electrolyte as ion sources.
2. Description of Prior Art
Electrochromic materials which manifest reversible variations in the light absorption properties upon application of properly controlled electric energy are well known in the art. Examples were disclosed in Talmey, U.S. Pat. No. 2,319,765 and Deb et al, U.S. Pat. No. 3,521,941.
Such electrochromic materials can be shaped in a desired pattern to display desired characters, symbols and patterns by reversibly selecting the ligh absorption properties through the use of the electric control. Examples were disclosed in U.S. Pat. No. 1,068,744 and the above-mentioned Deb et al, U.S. Pat. No. 3,521,941.
There are three types of ECD cells using transition metal oxides. The first one is the liquid type which includes a liquid electrolyte mixed with coloration materials (Jones et al, U.S. Pat. No. 3,283,656). The second type includes an inorganic insulation film as the ion permeable insulator see (the above-mentioned Deb et al, U.S. Pat. No. 3,521,941). The last type includes solid state electrolytic materials see (Castellion et al, U.S. Pat. No. 3,721,710). The electrochemical display cell of the present invention relates to the type which employs the liquid electrolyte as ion sources.
Various basic structures of an electrochromic display cell of the liquid electrolyte type have been proposed. Electrochromic materials are well known in the art, as already discussed above, and disclosed in, for example, the above-mentioned Talmey, U.S. Pat. No. 2,319,765 and the Deb et al, U.S. Pat. No. 3,521,941. Preferable materials are a thin film of amorphous tungsten (WO.sub.3), and a film of amorphous molybdenium oxide (MoO.sub.3) as disclosed in M. D. Meyers et al, U.S. Pat. No. 3,708,220.
Display electrodes of the seven-segmented type for displaying numeral information were disclosed in R. D. Giglia et al, U.S. Pat. No. 3,827,784, wherein the WO.sub.3 film is formed on the display electrode comprising a SnO.sub.2 film doped wth As.sub.2 O.sub.5. A technique for protecting the edge portion of the electrochromic layer formed on the display electrode through the use of an insulating layer was disclosed in Eric Saurer, U.S. Pat. No. 3,836,229. A technique for covering the lead electrode portions with an insulation film was proposed in J. Bruinink, Pro. Sym. Sept. 29-30, 1975 at Brown Boveri Res.
A counter electrode comprising a graphite plate or a stainless steel plate coated with an electrochromic material was disclosed in R. D. Giglia et al, U.S. Pat. No. 3,819,252, and Witzke et al, U.S. Pat. No. 3,840,287.
The background of the display is formed by adding a pigment to the liquid electrolyte (the above-mentioned R. D. Giglia et al, U.S. Pat. No. 3,819,252), or by disposing an opaque plate, through which ions can travel, behind the display electrode (R. D. Giglia, U.S. Pat. No. 3,892,472).
Examples of the liquid electrolyte were disclosed in L. C. Beegle, U.S. Pat. No. 3,704,057.
(i) sulfuric acid aqueous solutions ranging in concentration from a 0.1 to a 12.0 molar solution; (ii) sulfuric acid solutions of propylene carbonate, acetonitrile, dimethylformamide and other organic solvents; PA0 (iii) strong organic acid such as 2-toluene sulfuric acid in propylene carbonate and other organic solvents; PA0 (iv) alkali metals, alkaline-earth metals or rare-earth metals, such as lithium perchlorate, lithium nitrate, lithium chlorate and lithium sulfate in an organic solvent such as acetonitrile and propylene carbonate.
A semi-solid conductive electrolyte was disclosed in the above-mentioned M. D. Meyers et al, U.S. Pat. No. 3,708,220. The electrolyte comprises sulfuric acid and gelling agents such as polyvinyl alcohol, polyacryl amide, ethylene glycol, sodium silicate and carbo-sil. Especially, the polyvinyl alcohol/sulfuric acid shows a preferable characteristic. U.S. Pat. No. 3,708,220 also says that the viscosity and the vapor pressure of the above-mentioned gel can be properly controlled by adding dimethylformamide, acetonitrile, propionitrile, butyrolactone or glycerine.
Another example of the semi-solid conductive electrolyte was proposed in U.S. Patent Application Ser. No. 41,153 (1970) filed by D. J. Berets et al. The electrolyte comprises a mixture of greasy lithium stearate, 2-toluene sulfuric acid and propylene carbonate.
The above-mentioned electrolytes have various problems. The largest problem is that the WO.sub.3 film is soluble in the electrolyte and thus the WO.sub.3 film becomes deteriorated by the electrolyte, for example, the transparency of the WO.sub.3 film is decreased.
When the electrolyte comprises the sulfuric acid or the organic acid, the WO.sub.3 film dissolves in the electrolyte when the cell is maintained at 80.degree. C. for seventy-two hours. Moreover, the hydrogen over potential of the proton in the aqueous solutions or the organic solutions in about 1.5 V. Therefore, the system must be driven by a voltage below 1.5 V. Accordingly, the acid electrolyte is not suited for ensuring the accurate operation of the electrochromic display device.
When the electrolyte comprises alkaline metals, alkali-earth metals or rare-earth metals in the organic solvent, the above-mentioned problems are fairly improved. However, the possibility of forming a solution of the WO.sub.3 film still remains.
In the above-mentioned U.S. Pat. No. 2,704,057, the electrolyte comprising lithium perchlorate (LiClO.sub.4) in acetonitrile was disclosed. However, this electrolyte is not suited for the practical use, since the acetonitrile boils at 79.degree. C. The electrolyte comprising LiClO.sub.4 in propylene carbonate has a wide range of the tolerant temperature, more specifically, -49.2.degree. C. through 241.7.degree. C. However, this electrolyte is not stable at high temperatures. More particularly, the pyrolysis occurs in the electrolyte when the cell is maintained at 80.degree. C., and the solution changes from colorless is destroyed. Other electrolytes including propylene carbonate described in U.S. Pat. No. 3,704,057 do not show sufficient conductivity.
In case where the electrolyte is gelled by PVA as shown in the above-mentioned U.S. Pat. No. 3,708,220, the cell is still unstable at a high temperature, for example, 80.degree. C. That is, the gelling of the electrolyte is not effective for the antisoluvation of the WO.sub.3 film.
The electrolyte comprising the greasy stearate does not show high conductivity. Therefore, the responsiveness of the system is not satisfactory.
An improvement for the problems of the soluvation of the WO.sub.3 film was disclosed in R. D. Giglia, U.S. Pat. No. 3,819,252, wherein the electrolyte is saturated by WO.sub.3. However, this is not practical, since it is not possible that only the WO.sub.3 in the electrolyte preceipitates and dissolves in the electrolyte even when the temperature varies.
A further advance for eliminating the above-mentioned defaults of the soluvation of in WO.sub.3 film was disclosed the copending U.S. Patent Application Ser. No. 877,884, filed Feb. 15, 1978 assigned to the same assignee, and entitled "ELECTROCHROMIC DISPLAY CELL". Of particular interest in the context of the present invention is the above U.S. Patent Application Ser. No. 877,884, whose disclosure is incorporated herein by reference. The corresponding West German Patent Application was filed on Feb. 16, 1978 under P 28 06 670.8.
The following is the gist of the U.S. Patent Application Ser. No. 877,884. An electrochromic display cell comprising a display electrode coated with a WO.sub.3 film, and an electrolyte filled in the electrochromic display cell as an ion source. The electrolyte comprises Y-Butyrolactone mixed with LiClO.sub.4, NaClO.sub.4 or LiBF.sub.4. The WO.sub.3 film is formed on the display electrode under the condition where the substrate is heated up to 250.degree. C. through 400.degree. C.
However, mass production of the ECD is rather difficult even by the last-mentioned U.S. Patent Application Ser. No. 877,884 because of the inevitable occurrence of variations in the response characteristics in the ECD.